But who should be concerned? Residents of the country, first and foremost, says Bivens. A massive external debt could possibly trigger an exchange rate devaluation, especially if a country relies heavily on imports, creating a situation where money will be more difficult to tax in the future, debts will be more difficult to repay with less valuable currency and issues of fiscal sustainability arise.
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The US has spent $3.5 trillion in fiscal 2010, while taking in only $2.2 trillion in tax receipts, creating a $1.3 trillion budget deficit, about the size of Canada’s economy, or about 10% of GDP. That is the highest ratio of debt to GDP since World War II. The White Househas proposed to spend $3.8 trillion for fiscal 2011. The federal deficit is at $13.9 trillion, approaching the size of the US economy....Moody’s said the nation's debt rating was “not under immediate threat,” although it did say the rating could be downgraded in 2013 if the fiscal position of the US did not improve....If the US lost its Triple-A rating, the country would have to spend more to borrow, creating further financial pain
Does the US Deserve its Triple-A? - FoxBusiness.com
Tracing the history of the national debt back through our history, the CBO finds that the national debt did not exceed 50% of GDP, even when the country was fighting the Civil War, the First World War or any other war except World War II. As you can see in the chart, the national debt declined sharply after World War II as the nation began paying off its wartime debt when the conflict was over.
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Beyond those gradual consequences, a growing level of federal debt would also increase the probability of a sudden fiscal crisis, during which investors would lose confidence in the government’s ability to manage its budget, and the government would thereby lose its ability to borrow at affordable rates. It is possible that interest rates would rise gradually as investors’ confidence declined, giving legislators advance warning of the worsening situation and sufficient time to make policy choices that could avert a crisis. But as other countries’ experiences show, it is also possible that investors would lose confidence abruptly and interest rates on government debt would rise sharply.
Congressional Budget Office - Federal Debt and the Risk of a Fiscal Crisis